I'm new on the boards but have been messing with our two power wheels for the last 6 months and found the community posts very helpful. I thought I'd share a unique modification that people may find interesting -- using dual motors in each gearbox.

It spawned out of the failure of a 1-week-only 15T gearbox that I'd put in my son's Hurricane. The vehicle was using a 24V battery and 3 step-down (DC/DC) converters to provide variable speed (6-20V) and current limit (40A max). Following a fairly long session of continuous driving (me chasing the boys firing a nerf gun at them), we heard some unusual noises and parked the vehicle. We found that the motors were too hot to touch (but still functional) but we had spun the 1st gear pinion shaft in the gearbox case.

"Stupid 1st gear" was my first reaction. Part of the problem here is that once the motor heats up it transfers a lot of that heat into the Nylon gear and either strips the teeth or overheat the shaft:gear interface.

Instead of buying a steel 1st gear, I thought I'd do some calculations on whether it was practical to just remove it. The 2nd gear is same pitch but has a wider face (6mm vs. 4mm). The reduction provided by the 1st gear is about 2.88:1 (72 tooth input, 25 tooth output). What if I used a 11T pinion on a 7-series motor and drove the 2nd gear directly? Since I'm an engineer, I put together a spreadsheet to analyze a few possibilities

For the Hurricane in its current state this would have been a disaster as the 40A current limit would cut the available torque at the wheels by too much, so I decided I'd try it on the still-stock-wired F150 that we had a round (the Hurricane ended up getting a Mabuch 8514 w/16T pinion and has been happy since @ 22V). I found a 7-series motor that pulled about the same current as stock but provided 33% more torque. Then I thought: Could I fit two of those in each gearbox?

The answer is yes! I drew up a template in a CAD program and drilled out the appropriate holes in some 1/8" alumnium sheet:CAD template:Backside view (motors wired in series):Front view showing both pinions:

The Nichibo motor I'm using has a longer shaft than the Mabuchi motors and so the extra offset wasn't a problem. The motors are wired in series and so the vehicle runs on 24V but pulls about the same current as stock 12V. Running the motors in series means that each one sees the same current and should deliver the same torque to the gear. Since they're both driving the same gear there's no load inbalance issues. eg -- it's not like the classic wheel-spinning open-diff behavior you get from the two sides on a stock power wheels in 1st gear.

Also, since the power and torque is distributed between two motors, the driven gear (2nd gear) doesn't have a lot of tooth load -- it's actually not much harder on those teeth than the stock setup would be. The 2nd-to-3rd interface obviously has a lot more torque on it, and I expect the gears downstream to be the ones breaking.

The spreadsheet projection was that the end result at 24V would be about 2x the speed of the stock setup, with 50-60% more torque. The speed matches up pretty well -- the F150 has smaller tires and the spreadsheet suggests that it should be the same as the Hurricane (16T/Mabuchi 8514 @ 22V) and they're even in a straight line. I don't have a way to really measure the torque but there's certainly no lack of it. The tires have road-bike tire rubber on them and it will spin those at launch and leave 2-3' long black marks on the pavement. There's no comparing the holeshot on the F150 with this vs. the Hurricane with the current limiter.

I've been running this for a bit over a month (probably 10hrs of actual drive time, mostly on sloped pavement) and had one gear failure: A tooth lost on a final drive gear. The "interface" gear -- 2nd gear -- so far looks good on both sides. I told me son it was ok to start it in 2nd rather than babying it, so we haven't been taking it easy on it

I haven't tried running 36V into it, but the motors should hold up fine. I would be concerned about the 3rd/4th gears in the box breaking; the fact that it's already spinning the tires may mean that I'm already hitting maximum stress though. When I get another spare box I may try this .

This could be done using two 5-series motors as well. Also, if I were doing it again I would have used a 14 or 15T pinion. The 11T is more reduction than is necessary given the torque of the motors. If you were using an ESC with limited current (say, 60A), you're already limiting the maximum torque and may want a smaller reduction.

BTW, if you look closely you can see that I'd previously tried a single-motor variant of this (there's a hole in the gearbox between the two motors). I had attempted to use a single Banebots 24V motor (Mabuchi 5033). That approach worked but didn't have any of the advantages of dual motors: higher torque, better thermals, etc. Alas, I didn't provide proper venting on the face of the motor and it overheated at 36V.

Got some video of this rig running on 36V. It's temporary -- the wife thinks it's too fast and I agree. I did not get any of the really good video -- drifting sideways at 10-11mph. Too dangerous in a truck like this, but a fun experiment.

F150 has hybrid plastic/rubber tires (will post in the Wheels section about this soon)

Since you've got me interested in this now... How much of a pain was it to drill all the holes in the right places and line everything up correctly?

If I were crazy and pretended I had enough time I'd probably do this mod with two VEX 775Pro's /w 15T pinions, a hefty 36V ESC and build myself a 10S 18650 Li-Ion battery pack to squeeze in a few extra volts @ 42V.

It wasn't too bad, but there are definitely a few more steps involved than the usual motor swap . I've started in on a second one using 16T gears, in a 19T small-axle gearbox for the Mustang, and I'm taking more pictures of the process. The first one was a good learning experience -- this iteration is to try to make it more streamlined.

The nice thing about using an aluminum plate is that you can oversize the holes for the motor shaft and the motor screws by a small amount and then move the plate around until you get the spacing you want for the motor pinion vs. the plastic gear. The motor's snout is inserted into an exact fit hole in the alumnium, so it moves with the plate. Once you're happy with the alignment you tighten down the motor retaining screws (with washers under them to spread the load out). I then drilled additional holes through the gearbox and aluminum further away from the motors, and installed machine screws. This ensures the plate can't move with respect to the gearbox case, and it also stiffens up the gearbox case itself. You can see these in the picture that shows the back of the motors/gearbox -- they're the three nuts tightened up against the aluminum in a triangle pattern (one is also visible in the front-side view as the flat-head machine screw; the other two are underneath the gear). My main concern was ensuring the plate was fixed in place relative to that 2nd-gear shaft.

Later if you decide you want a different gear ratio, you can -- within limits -- just loosen up the screws and move the plate around, then retighten in the new position. The motor doesn't really create that much torque, so if you have a screw that is 1-2" from the origin of the torque (eg. center of motor), it doesn't need to exert a lot of force to prevent things from moving.

I'm hoping to get more done this weekend and should be able to post more of a step-by-step guide.

I'm really expecting the limit will be the interface between the shaft and the hub for one of these gears -- they're going to be spinning 3-4-5x the stock speeds and that should put some serious heat into them. So far I haven't had any indications of problems but I am still only maxing out at 12-ish mph on a 13.5" tire -- with the Mustang that gear's rotation speed will be 45% higher. Since your 2nd gear apparently lasted quite awhile running similar RPMs, maybe this isn't really a limiter. We'll see .

Awesome, a step by step guide would be great...would definitely lessen the unintended screws ups and trying to do it for the first time.As for my 2nd gear(s), we haven't had a single problem with one yet. Also.. I have to ask two questions. would you mind sharing your magical engineering spreadsheet?? and do you have a write up or anything on this homebrew motor controller of yours?

Every once in a while somebody goes that new direction and takes things to a totally different level. I am happy to see another one of those moments here and now. Big thumbs up on several levels with this one. Thanks for sharing. I'm sure you have inspired others with this pioneering modification. Great...just great.

Impressive! I'm using a 24V 1000W ESC with 2x 775s (16T pinion w/ steel 1st gear) in my son's F150 Raptor and so far not overly impressed with it for off-roading. I may have to try your "quad" 775 route if further modifications don't provide the torque I'm looking for.

As a side note something you may consider doing is what I did to the wheels - see the photos below. I'd just do the rear tires as I'm running into rubbing issues with the front. The tire rubs the body (not even the fender well) to the point where it completely stops the truck. This only happens though when turning on a hill. Unless you have a mod in mind to prevent it that's why I say just do the rear for now. I'm probably going to either widen the wheel base or raise the front an inch or two. The tire size is 16x8.00-7 and here's a link to the tires I bought https://www.amazon.com/gp/product/B06X9Y2YL3/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1. Sorry Amazon Cloud's Photo service doesn't allow me to embed photos so I can only share them using a link.

I don't have anything written up on the homebrew controller yet. It's a modification of the "DIY ESC with no features" that was posted on the board a few years ago -- higher frequency (3khz), overkill MOSFETs (4xIRFB4110), simpler overcurrent protection. I do want to get it posted sometime soon, when I have more time for writeup .

As for the calculator: The old version was very crude and not really external-friendly. I spent a few hours last night and this morning creating a better one. This also builds estimates of how much heat is generated in the motor when running, estimates peak power output, and attempts to model things like wiring resistance. It's not perfect but should provide some useful guidance.

I'll have to get back to you on the minimum shaft length measurement. In the original variant I used 1/8" thick aluminum plate but the motor hub is slightly deeper (~0.175") so that meant I had to remove some plastic on the gearbox case where the hub hits it. I used a Forstner bit to shave some of the material off in the right spot but it's messy. Ideally I'd just drill a ~1/4" hole where the pinion shaft goes through, so the new ones I'm making have a 3/16" plate so that it completely holds the hub -- but this moves it another 1/16" away from the gear. For the 7013F motor I'm using, that should be no problem, but it may make "short shaft" motors more of a problem.

But I'm a bit reluctant to suggest a solution that requires paying $50 to get 4 pinions . I got 4 sets of 5 different pinions from China for less money (13/14/15/16/17T -- now I can do relatively easy ratio changes)...

Also, I need some time to really run a few more scenarios through the calculator. I think the 775pro looks like a great motor with the ball bearings and high efficiency. However, in a dual-motor configuration maybe the cheapo 7013F motors (which have a 13mm long shaft) will actually hold up just fine. 18V each runs them at 23k RPM -- they'll probably go forever even under that higher load.

I also want to a basic calculator in the spreadsheet to help people estimate how much net power is required when going up a hill (easy to calculate given the weight of the vehicle, grade %, and speed) and allow some estimation of drag for different surfaces. The latter is going to require me to improve my instrumentation on the vehicle; I don't currently have anything that displays the current draw while driving, but I have a 100A/60V display sitting on my workbench for that purpose. Then I can collect stuff like:(A) Free current of motors out of gearbox (~2.2A for the 7013F @ 12V - less than the 3.2A spec)(B) Current used installed in gearbox, in vehicle (3.0-3.1A per gearbox with wheels off the floor, @ 24V in the dual-motor configuration(C) Current used on flat pavement with stock plastic tires (I don't have stock plastic tires anymore on any of my vehicles)(D) Flat pavement with rubber tires(E) Sloped pavement with rubber tires (to help verify the calculation above)(E) Flat short grass(F) Similar measurements at different speeds to verify things are linear (or find out which ones aren't).etc.

I think it would be great if people had a way to estimate the amount of net power they need, which could then be used to look up (on the spreadsheet) an estimate of how much heat/waste is generated inside the motor, and then compare that against the recommended maximum power dissipation per motor (eg. maybe 50W for RS550, 100W for RS775).

Yea, the cost of the long pinions does present an issue x 4. However, since I like to go overboard on these things I'd feel like I cheaped out if I didn't do the 775Pro's in a dual motor setup. Otherwise--how would I ever know how fast we could get it to go? I wish I could find a heatsink & fan setup for a 775 motor. I haven't come across any.

do you put any emphasis on getting Hardened Steel Pinions? I noticed the long pinions on amazon didn't mention being hardened. I've been using the Robinson Racing 87xx on everything...definitely let me know if you get a minimum shaft length measurement.

also, I've got a dumb question for you. I'm a little confused about why you chose to wire two motors in series. I'm sure i'm missing something but here's the basis of my thought process. When I think about two 8 ohm speakers being wired in series, I know that doubles the resistance to 16 ohms collectively . If the same speakers were wired in parallel, it would cut the resistance in half equaling 4 ohms total. Of course the 4 ohm setup will be louder, as the amplifier sees less resistance and subsequently delivers more current. This is contingent upon the amplifier being capable of driving a 4 ohm load without overheating itself. So my question is...if I've already got a "beefed up" 24V ESC controller-- why not wire the motors in parallel to reduce resistance and drive more power? Is it out of concern that most if not all 24V controllers would burn up trying to keep up with the current demand of the motors?

Like I said, I'm sure there's something simple I'm missing. I know my audio/speaker analogy only goes so far to draw a comparison. I looked back at my notes for modifying the controller. The circuit board originally had 20A components and I replaced them with 60 amp schottky rectifiers & 120V 179A N channel MOSFETs

Ah, that's a good question. Let's say I built one of these out of two of my Nichibo 7013Fs. They have a rated stall current of 65A @ 12V and put out about 530mN*m of torque at that current. The DC resistance of the motor is around 12V / 65A = 185milliohms (this will become important later).

If I perfect wires and an unlimited current driver, it wouldn't matter. I could put them in -parallel and they'd draw a total of 130A @ 12V -- 65A to each. Each one would put out the rated 530mN*m of torque and so the total torque applied to the 2nd gear would be equal to 1260mN*m torque multiplied by the gear ratio (2nd gear # teeth / motor pinion # of teeth).

If I chose to wire it to 24V, it would pull 65A @24V. It's the exact same amount of power, and each one puts out the same amount of torque, so the net result is the same.

Now let's add some wire resistance. If I have a total of 8' of 12ga wire between the battery and my motors (4' each way), that adds about 13mohms of resistance. Let's throw in a few switches (typically rated at 75mohm max for a 30A switch). Let's say the whole thing runs up to just 30mohms. What happens now when I try the two scenarios.

For the parallel case, my total DC resistance is the motor resistance (185mohms || 185mohms = 92.5mohms) plus my wiring/contact resistance (30mohms). This equals 132.5 mohms. At 12V, this will now pull 90.5amps. This is a lot less than the 130A it pulled before. The torque will be down (to 90/130 * previous torque... about 69%), and the wire and contacts will burn approximately (I^2*R) 243watts at the stall condition.

In the series case, my total DC resistance is the motor resistance in series (185mohm + 185mohm), plus the wiring/contact resistance (30mohm). This is a total of 400mohms. My total current at stall will be 24/0.400 = 60A (vs. 65A originally). I've still go >90% of my torque, and my wiring only burns 108 watts.

To get the same performance as the 24V/series, you'd need a total wire+contact resistance of 8mohms -- that's some massive wire and much better switches.

With the series connection I can use cheaper mosfets/switches, lighter wires, and I waste less of the electricity in those components.

If you're current-limited by an ESC, the difference is huge. With the motors in parallel and a 50A current limit, each of those motors is going to get 25A. That's ~200mN*m of torque per motor. In series, they each get the full 50A. That results in double the torque. This makes sense, since the ESC is pushing more power (12V * 50A vs. 24V * 50A). But the ESC is likely only a bit less efficient at 24V than it was at 12V. So now you can either get a cheaper ESC (25A limit) or just enjoy the extra torque.

I do a lot of audio design too -- speaker cabinets, class-D amplifiers, frequency-shaping circuits, etc.-- MHO, it's easier on typical AC-power amplifier to get more power with higher voltage, at least to a point. My favored class-D driver is the IRS2092, which has maximum of 200V rail-to-rail, so +/-80V supplies is the sweet spot. You can pair that with a fairly cheap FET. I'd rather connect something like a 2+2 (dual voice coil) speakers in series and run the amplifier bridged to get double the voltage than try to push a 1ohm load.

That said, I see a lot of car audio systems that use low-impedence setups. One big difference there is that you don't start off with high voltage. If you have to always boost the power supply from 12V up anyway, there's a limited benefit to boosting it to 120V so you can use a high-impedence driver. In the power wheels case, if you're putting in 2 or 3 batteries anyway, then try to find a setup that works at high voltage. You don't get any more efficiency out of the battery, but you have lower losses everywhere else.

Other questions: Hardened pinions? I don't think I would be concerned in this case since I'm driving the plastic 2nd gear anyway. Definitely a requirement if you're going to use them with a steel gear (part of the reason this mod exists is because I was too cheap to buy a steel gear, and thought this was potentially a better solution). I do think the Hot Racing ones are hardened (it claims to gone through Nitriding, which is one of the standard hardening processes for steel)

BTW, I don't know why MLToys doesn't t offer two "different" steel gears. They could make a variant with 4 fewer teeth (on the large gear) and then people could run +4 motor pinions, eg. 26T in a 21/22/23T gearbox, or 20T in a 15/16/17T gearbox and get another 15-25% speed out of it... I guess there's always the question of demand, but if you really want speed that's probably easier on the motor.

Shaft lengths will probably come tomorrow or the next day, depending on evening commitments. Also, what I'm doing right now is building a 1/4" thick template plate for the 19T gearbox case. This has the pilot holes in the appropriate locations and I'm hoping I can: (A) Make as many identical motor plates as I want from that (use the template to get the pilot holes, then drill out to the larger size for the motor hub and motor vent holes), and (B) clamp the template to a gear box and use to drill the pinion & M4 screw holes in the gearbox (then you put the regular plates on and install the actual motors). If all goes well, I'll make 2 or 3 sets and maybe I can send them out for some additional beta testing. Unfortunately I'm using a drill press, not a fancy CNC mill or routing table, so we'll see exactly how accurately they come out.

I'm embarrassed to say how long it took me to wrap my head around your explanation. I had to do some digging to fill in my knowledge gap but I think I've got it...correct me if I'm wrong though.

So, it really has to do with the nature of how a series circuit works...because the overall effect is that the current is equal through all components--you literally need half the amount of current than i would (in parallel) in order to drive two motors. resistance increases in series but it's a negligible consequence compared to the benefit only needing to deliver half the current...Leaving you with a more reasonable chance of being able to drive the motors at their full potential. The other consequence of series is that the battery voltage (24V) will be shared by each motor, so each motor will only see 12v. However, as you stated--it's easier to supply more voltage than it is to deliver more current. Do I have all that right ? Watch, I probably still have it wrong.

Although, I will say my current wiring & contact resistance *should* be very low. My wires are much shorter because my batteries, motors and ESC is very close together underneath the seat. I also went overboard on wire gauge size. I estimate 4 feet of 8 gauge wire having a resistance of 3mohms + the contact resistance of 2 relays (no idea??) so for kicks I just halved your example of 30mohms...called it 15mohms due to my short runs of massive wire and added 92.5mohms for motor resistance = 107.5mohms. @ 12V it would now pull ~112amps which is approximately 86% of the original 130A. better than 69% but less than the 92.4% of amperage you managed to keep by wiring in series. Looks like I'd be better off buying the 36V or 48V version of the same controller I have and doing the same sort of modifications that i did to my current one. That way I wouldn't have to change any of my wiring--just connect it up and go. now to the next problem...the added weight & cost of more batteries. ugh!

Yeah, that's pretty much it. Good wiring goes a long way to reducing the waste... A large part of my motivation was to keep the torque up without having to buy super-high-current drivers. I'm running 4x IRFB4110 which are 120A (package-limited) MOSFETs -- it could certainly be called overkill. The typical 500 and 1000W ESCs have 30-40A limits and would really struggle with a parallel arrangement -- I was only discussing *one* gearbox in the previous post, and you normally have two in parallel (trying to pull 2x the current), and some of that wiring is shared so you can think of it as counting double. Try to pull 200A through one of the stock shifter switches or even an upgraded relay...

When driving 2nd gear you have to really internalize the fact that you've lost ~2.8x gear reduction. You have to make that up somewhere OR be happy with slower acceleration. By serializing the motors you get most of that back (you just doubled your torque for the same current), and can then tweak the gear ratio to balance the speed vs. torque you want.

When you moved from a 19T (?) gear to 21T and switched to that 9009F you lost only ~20% torque for the same current. if I go from a 19T stock gearbox to a dual-motor gearbox with a ~15T gear on each motor, I have basically a 2x gear reduction vs. stock; now with dual motors I have the same final driven torque and 2x the speed (as long as I ran it with 24V) and the same heat output (in each motor) as the stock setup. Upgrade to slightly torquier motors (even cheap ones like the 7013F) and that's fairly livable with a 40-50A ESC and it will barely get warm.

Those should work alright -- do you have a rough idea of where the overcurrent protection is set to with your modifications?

I finally did some measurements this morning.

The 775PRO Vex motors should be very similar to the Mabuchi short-shaft ones. If I used my current 5mm aluminum plate, you'd end up with 25mm from the face of the motor to the end of the 2nd gear teeth. With a 14mm long pinion gear and full engagement to 2nd gear, that means you have 11mm from the motor face to the start of the pinion -- which is only 1.7mm of engagement on the motor shaft which isn't going to be enough. With the Nichibo motors the same setup would give you 8mm of engagement which should be plenty.

Ways to mitigate:(A) Buy the "long" pinions, which appear to be 22mm total length, giving you 8mm more engagement. Problem solved, with $$$ (B) Don't engage all of gear #2. In the current gearboxes in the truck, the 11T Associated Racing pinions I had (which I will not buy again for this application) have only ~4-5mm of usable gear teeth. The profile changes around 5mm as it transitions into the body of the gear. The Hot Racing don't have this (they have a hard step, but the gear is around 6mm). 4mm of tooth engagement (same as the full thickness of 1st gear) has been enough that it doesn't break, but it does wear the gear unevenly. This could buy you 2mm.(C) Go to thinner plate and drill out/relieve the gearbox to accommodate the (now-protruding) snout. With 0.100" aluminum, that would buy you another 2.5mm.(D) shave the bottom of 2nd gear 1mm

If we used the thinner 0.100" aluminum (~and you backed off slightly on the 2nd gear fit, that could gets you to 6mm of pinion engagement on the motor which should be sufficient. I'm not extremely confident on the 2nd-gear life, though I wouldn't expect it to fail quickly there -- 1st gears will generally handle this kind of torque as long as the gear doesn't get too hot.

The long pinions would be the most robust solution, at the added expense of the pinions themselves.

Another option is to get a Johnson motor. Based on their catalog it looks like most of their LP785 motors have a 16.5mm length (face to end of shaft) -- which should give you 5-6mm of engagement with standard pinions even with the 5mm Al plate, and they're using the same (17.5mm) hub diameter, so it would fit in the same template.

The HC785LP-001 is similar to the Mabuchi 9013 (more like an "8013" motor based on the stall current -- thinner wiring than the Mabuchi) -- 21000 RPM @ 18V, 100A stall. In a dual-configuration you maybe could use the one you'd linked before (the HC785P-001, 755-sized case, 12V @ 21000 RPM), though the efficiency is a lot worse (64% vs. 77%).

Unfortunately I don't know where my overcurrent protection is set to. Since I did a current reference bypass on the circuit with a 1k trimmer, I just "set the dial" to slightly above half. My thought was to put my amp clamp on the motors and start with the wheels spinning freely in the air--then add some resistance while watching the meter and see where the controller stops it. I don't know much (at all really) about comparing MOSFET specifications but after comparing both of ours I suspect that yours is much more capable of handling heat in general.

I think the long pinions would be the best solution. Maybe if I dig deep enough on the interwebs I can find a better price, we'll see.

One other odd observation: I noticed that Mabuchi's CURRENT specifications for their main 775 lines (8016/8514/9013) shows a 15mm front hub diameter (vs. 17.5mm) and a 18.2mm dimension from the motor face to end of shaft. The picture on this ebay listing (Ebay 8514 listing) looks like this as well -- the hub is smaller and the shaft is longer than the Banebots 18V (labeled as an 8514 motor also) that I measured. The Banebots 24V motor (Mabuchi 5033, I think) has a 15mm front hub but the same short shaft (12.7mm).

Even the listings for the 9511 "custom" ones are split. Some show a short shaft and some have long ones:

Check this motor out. 775 12v @ 23,000rpm. it's a double shaft motor and the front shaft is 20mm long. unfortunately there are no torque specs and it's not a ball bearing motor. However, it is pretty affordable. Might be good candidate for quad damage setup?

These "795" sized motors are interesting, but again difficult to really model without a real datasheet. Longer case size theoretically should improve torque/amp by a few %, but there's no information at all about stall torque or amperage.

With the quad damage setup, you need about 22500 RPM if you want to run a 14"-wheeled Jeep at 20mph on a 17T pinion (the template I built should span 13-17T). I've been going after that running 36V series with the cheapo 7013F motors (23.2k RPM @ 18V), but I supposed you could get higher-speed motors and run them in series on 24V. I'm figuring on trying to use my stash of pinions (4x13T, 4x14T, 4x15T, 4x16T, 4x17T) on quad damage gearboxes and calling it good , though the application to the Hurricane is going to be a bit difficult. Even with the 13T gear at (just) 24V, the 15.5" tires means you're doing 12mph with the 7013F motors. The current box in the truck has a bunch of 11T pinions and 13.5" tires (lots of torque).

I'd run THOSE in parallel -- since they're all torque (about 5x the torque of the 9009F, per amp). WIth such a low stall current, you could make it a direct drop-in w/o ESC or anything -- just wire 24V up and go, and you'd get 2.5x the speed and 30-40% more torque (at the wheels) as a stock power wheels, and never worry about it overheating.

But in the end they're not really more efficient and they're not cheap. If I were trying to do this on the cheap, I'm back to the 7013F motors .

Built a 16T quad-damage box for the Mustang and I've decided it's too fast for what we do around here, at least without a variable-speed controller. At 24V it's running about 11mph. We briefly ran it at 36V, which was absurd in the cul-de-sac. Unfortunately it was dark and the kids won't drive it again at that speed, so the video quality is pretty lousy.

I think I will likely open it back up and put 13T pinions in until I get another motor controller built.

Also, picked up 6 new gearboxes plus a few wheel drivers for $120 and have 20 more 7013F motors on order, so I can make 3 sets with the pinions I have left after outfitting all my vehicles (likely will have 14/16/17T left, if I put 15T in this one long-term).

And finally got a 2nd speed controller built -- video with a 14T quad damage box in the Mustang. Still not sure whether to go back to 16T. The "extra" burnout power is fun, but the speed worries me quite a bit. Sliding sideways into the curb -- even the low/sloped gutter curbs we have -- could flip the vehicle, even with the batteries (and weight) under the driver.

We'll see how long they last. The original 11T box in the F150 is still running fine, even with multiple full-power launches on pavement with mountain-bike tires on them (no slip) -- though that is running at 24V.

Update on this one: Both vehicles are still running strong with no failures. The truck is running 24V continuously (2x18AH) and has been running with mountain bike tires over the stock tires (not hybrid). These don't slip and that makes for pretty strong launches on asphalt. I haven't even cleaned the dirt off of the gearboxes cases (from the "OHV park" run we did last November, which was all done at 36V).

The Mustang is still running the plastic/road-bike hybrid tires, 36V, 1st-gear bypass and 14 tooth pinions, and I haven't touched the gearbox since I swapped the 16T for 14T (which was around the time I put its motor controller in). It's had a few "over-spec" riders, including two brief stints with 200-lb me driving it (just a few minutes... ). I did recently upgrade the batteries to 6x9ah (3s2p) from the previous setup (1 stock battery + 2x7.5ah, all in series); the previous setup was sagging to 25-27V going uphill at full throttle with heavier riders (ahem... me). No such problem now and it will spin the tires even with the wife driving it. .

I still have 6-8 gearboxes and some aluminum plate for the next time I feel like spending a few hours building them up. The biggest problem is that honestly they're just too fast at 36V, at least for the cul-de-sac area we have. Out at the OHV park 36V was a perfect fit for the truck -- it just needs real rubber tires (cushion) and suspension.